Nuclear vs. Diesel Submarines

Oct 18, 2011

By Jonah Friedman

In recent months there have been a number of calls for building diesel-powered submarines for the United States Navy. In two separate Defense News articles in June and September Gary Schmitt and J. Scott Shipman, respectively, urged the Navy to build such vessels. They cite a number of advantages for diesel subs, both operationally and in terms of cost. However, while diesel submarines may enjoy some advantages over nuclear bots, the significant drawbacks to this form of propulsion should be kept in mind as well.

Schmitt and Shipman highlight some of the most important benefits of diesel submarines compared to their nuclear-powered counterparts. Perhaps chief among these is cost. They both note that one of the biggest challenges facing the Navy today – and one which it will continue to face in the coming years – is a lack of submarines in sufficient numbers to maintain a presence in areas of interest to the United States. Shipman in particular points to the need to augment the U.S. naval presence in the Indian Ocean to deal with piracy, and in the western Pacific to monitor a rising China. Leaving aside the question of how effective submarines would be in fighting piracy or how many are truly needed in the Pacific, the concerns surrounding a shrinking navy is a legitimate one. One of the chief causes for this contraction is the extremely high cost of nuclear-powered submarines. As Schmitt and Shipman note, the cost of a Virginia-class submarine is in the area of $2 billion, whereas the cost of a diesel boat is around $500 million. Given these prices, the U.S. Navy could certainly procure more submarines (and have a correspondingly greater presence around the world) if it pursued diesel instead of nuclear versions.

The other major benefit conferred by diesel subs relates to their operational capabilities. While early diesel technologies greatly impinged on the length of time a submarine could remain submerged and deployed, new technologies have improved this time. Through the Second World War, submarines needed to either surface or use snorkels in order to obtain the oxygen needed to recharge their batteries and continue operating. This both left them vulnerable to attack and reduced their range, since they could only be submerged for several days at a time. Modern diesel submarines utilizing air-independent propulsion can remain submerged for about a month. Moreover, as Schmitt points out, unlike a nuclear-powered sub, a diesel sub can turn off its engine and sit on the ocean floor “deadly silent, while monitoring whatever passes over and around it.” (Although it should be noted that a nuclear sub could also switch off its propellers and also remain extremely quiet)

Finally, proponents of diesel submarines note the large number of other states which use diesel subs, and the potential for the U.S. to enter this market and benefit economically. Both Schmitt and Shipman argue that our allies who use such vessels would be happy to purchase U.S.-made diesel subs. Shipman in particular cites the benefits for the domestic economy and the revitalization of industrial infrastructure that a boom in diesel submarine construction would create.

While these advantages are notable, the disadvantages of diesel subs (at least as compared to their nuclear counterparts) are significant. Although the cost of nuclear submarines per unit may be more than diesel boats, the numbers given by the latter’s proponents may not be realistic. While we have a good idea about the cost of, say, Virginia-class subs (since we have already built several of them), diesel boats have not been built in this country in decades. Shipman notes that it is possible that the $500 million estimate for a diesel sub could be overly optimistic given our current procurement practices. He cites some Navy sources that have put the cost of a diesel submarine at about $1 billion. This would still be cheaper than a nuclear version, but significantly less so.

Operationally, there are advantages which nuclear subs possess which cannot be matched by diesel boats. The most obvious of these relates to the amount of time they can remain submerged and deployed. Whereas air-independent propulsion technology allows submarines to remain submerged for a few weeks, there is no limit to the amount of time a nuclear submarine can remain submerged (barring the limits of the crew). This allows for nuclear subs to be deployed and submerged for far greater periods than diesel boats, and it has implications for their operational performance as well. While the Navy could build more diesel subs to enhance its presence in certain parts of the world, those subs would necessarily need to refuel and recharge more frequently than their nuclear counterparts. The amount of time they could spend completing their missions (such as reconnaissance or tracking other submarines) would therefore be reduced. Although there would be fewer subs in the fleet if they were all nuclear, these vessels would be able to devote a greater proportion of their time to their missions, and they would be less vulnerable and more efficient. Additionally, the greater power output provided by nuclear reactors allows nuclear submarines to travel significantly faster than their diesel counterparts.

With regard to the interest of other states in purchasing U.S.-built diesel subs, it may be that there is a market for such vessels in which the U.S. could excel. However, it is also true that other states see the value of pursuing nuclear propulsion for their submarines, and have been making efforts to develop it. The Nuclear Threat Initiative cites Brazil and India in particular as enthusiasts for nuclear-powered subs, and notes that India plans to eventually build about half a dozen of them as strategic nuclear weapon platforms. Argentina is another state which has recently announced its intention to develop nuclear submarine propulsion. Shipman mentions that “modern diesel submarines have proliferated over the last 25 years,” and that “as many as 39 countries have diesel boats.” The implication might have been that the proliferation of diesel submarines shows that other states have recognized their advantages while the United States continues to ignore them. Yet it seems clear that at least some of these states (such as the ones mentioned above) would have nuclear propulsion for their submarines if they could do so.

It is important to keep these disadvantages in mind, as well as the significant benefits derived from nuclear propulsion, and to consider the most appropriate uses for diesels. Diesels are best used in areas closer to shore, and their comparatively shorter range and endurance make them somewhat less suitable for long-term reconnaissance missions or for traversing vast expanses of open ocean to fulfill their missions. Their slower speeds might make them more appropriate for defending against other vessels closer to their bases. The need for the U.S. Navy to project power around the world limits the usefulness of diesel subs, given these drawbacks.

The Navy’s nuclear submarines provide both a virtually invulnerable deterrent force in the form of its missile boats, and a persistent attack capability in the form of its attack subs. These assets should continue to form the core of the U.S. Navy’s submarine fleet. Yet Schmitt and Shipman are correct in their assessment of the strategic challenge the Navy faces in ensuring it has enough vessels to fulfill its tasks. An alternative to the diesel option (mentioned, but not favored by Schmitt) might be unmanned underwater vehicles (UUVs). These would be cheaper than building more nuclear subs, and would be able to fulfill “’dull and dangerous’ missions” that are currently done by some attack submarines. Although the effectiveness of UUVs have yet to be fully realized, investment in this area may make more sense than building a new fleet of diesel submarines. A realistic and balanced assessment of the capabilities of various alternatives vis a vis nuclear subs should yield a better sense of which benefits can be achieved by their adoption.

Jonah Friedman is a Research Intern for the Project on Nuclear Issues. The views expressed above are his own and do not necessarily reflect those of the Project on Nuclear Issues or the Center for Strategic and International Studies.